1. Field of the Invention
The present invention relates to a method for manufacturing a GaN-based semiconductor optical device, and more particularly to a method for manufacturing a GaN-based semiconductor optical device in which the waveguide has a window region formed in an end face thereof.
2. Description of the Related Art
Recently, in order to enhance the recording density of optical discs, intense R&D effort has been carried out to develop semiconductor lasers capable of light emission in the blue to ultraviolet range. Such semiconductor lasers include GaN-based semiconductor lasers formed of a Group III-V nitride compound semiconductor such as AlGaInN, which are already in practical use.
These blue-violet laser diodes (hereinafter referred to as “blue-violet LDs”) are formed by growing a compound semiconductor in crystal form on a GaN substrate.
A representative compound semiconductor is the Group III-V compound semiconductor, in which Group III and V elements are combined together. Mixed crystal III-V compound semiconductors having different compositions can be formed by bonding pluralities of Group III atoms and Group V atoms in different manners. Examples of GaN-based compound semiconductors used to form a blue-violet LD include GaN, GaPN, GaNAs, InGaN, and AlGaN.
As is known, GaN-based LDs are used as optical sources in next generation DVD drives (a type of optical disc drive). In order to increase the writing speed of these drives, it is important to increase the output power of the GaN-based LD used therein. One problem limiting the output power of LDs is catastrophic optical damage (COD), since it greatly affects the reliability of the LDs.
An effective method for preventing COD to an LD is to reduce the light absorption and temperature of its end face portions.
The end faces of an LD, unlike the inside of the LD, include a large number of dangling bonds (or broken covalent bonds), since these faces are formed by cleaving. Therefore, surface level, that is, surface energy states which trap carriers are present at the end faces of the LD and cause many nonradiative recombinations, i.e., recombinations of electrons and holes which do not contribute to light emission. Since carriers injected into the end face portions of an LD are lost due to nonradiative recombination, these portions have a lower effective density of injected carriers than the center portion of the length of the LD in the waveguide direction. As a result, the end face portions act as light absorbing regions and increase in temperature during the operation of the LD, since the absorbed light is transformed into heat energy. This results in a reduction in the bandgap, which causes these portions to further absorb light. This positive feedback (or vicious cycle between the temperature rise and the light absorption increase of the end face portions) may lead to a situation where the temperature of the end face portions exceed the melting point of their material resulting in melting of these portions and hence occurrence of COD. One method for preventing this is to form a window structure in each end face portion to increase its bandgap and thereby reduce or limit its light absorption.
Another method for preventing COD to an LD is to enhance the heat dissipation capability of the LD and thereby reduce its temperature.
Conventional LDs formed of GaAs-based, AlGaAs-based, InP-based, or AlGaInP-based material have a window structure formed in one or each end face thereof. Such a window structure is formed by disordering an end face portion of the active layer and thereby increasing its bandgap. That is, the formation of the window structure reduces or limits the light absorption of the end face portion.
A known conventional semiconductor laser device having a window structure is constructed as follows.
An n-Al0.6Ga0.4As lower cladding layer, an active layer region, and a p-Al0.6Ga0.4As upper cladding layer are sequentially formed on top of one another on an n-GaAs substrate. (The active layer region includes an undoped GaAs active layer.) Electrodes are then formed, and the wafer is cleaved to produce a Fabry-Perot resonator. An SiO2 film having a thickness of 2500 Å is then formed on the light emitting end face of the resonator and heat treated at 950° C. in a hydrogen atmosphere for 30 seconds by RTA (rapid thermal annealing) to disorder the active layer. This increases the energy gap of the active layer, thereby forming a window region therein. (See, e.g., JP-A-6-13703, paragraphs 0009 to 0014, FIG. 1.)
A known method for manufacturing a semiconductor laser device proceeds as follows. First, a laminated semiconductor structure, or multilayer structure, is formed on an n-type GaAs substrate. The laminated structure includes an n-type AlGaAs cladding layer, an active region including a GaAs quantum well layer, and a p-type AlGaAs cladding layer. Next, a mesa, or stripe, is formed by etching and then buried under an n-type GaAs layer to form a waveguide. The wafer is then cleaved in a direction perpendicular to the length of the stripe to produce a resonator having a length of approximately 500 μm. A silicon dioxide film is then formed on each end face of the semiconductor laser device structure (or resonator) and heated at 850° C. for 30 minutes to diffuse Si into the end face portions of the quantum well active region. This disorders and thereby transforms these end face portions into window structures having a wider effective bandgap than the undisordered center portion of the active region. (See, e.g., JP-A-6-104522, paragraph 0015, FIGS. 1 and 2.)
Another known method for manufacturing a semiconductor laser device proceeds as follows. First, a plurality of semiconductor laser devices are formed on an n-GaAs wafer. Next, the wafer is cleaved along crystal planes into laser bars each including a plurality of semiconductor laser devices arranged in a line. It should be noted that this cleaving forms the end faces of the resonators of these devices. A reflective film made up of an SiO2 film and an SiN film is then formed on the front end faces of the resonators. The reflective film of each laser bar is then lamp-annealed or laser-annealed in an RTA system to cause the excessive Si in the SiO2 film to diffuse into the front end face portions of the semiconductor laser devices. This causes the front end face portion of the MQW active layer to be intermixed with the Si, resulting in an increase in the bandgap of the MQW active layer and hence a decrease in its light absorption. (See, e.g., JP-A-9-92927, paragraphs 0024 to 0027, FIGS. 1 to 5.)
Further, there is a known technique of achieving selective quantum well intermixing in GaAs—AlGaAs structures by impurity-free vacancy diffusion. (See, e.g., Boon Siew Ooi et al., “Selective Quantum-Well Intermixing in GaAs—AlGaAs Structures Using Impurity-Free Vacancy Diffusion,” IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL, 33, NO. 10, 1997, pp. 1784-1793.)
GaAs, InP, and AlGaInP have relatively low heat conductivities of 0.55 W/cm*K, 0.68 W/cm*K, and 0.067 W/cm*K, respectively. On the other hand, GaN-based material, which is used to form a blue-violet LD, has a high heat conductivity, as compared to GaAs-based, InP-based, and AlGaInP-based materials. For example, GaN has a heat conductivity of 2 W/cm*K. Therefore, blue-violet LDs are less likely to suffer from COD than LDs formed of GaAs-based, InP-based, or AlGaInP-based material, and conventional blue-violet LDs do not have a window structure formed therein.
However, since the end faces of the resonator of a blue-violet LD are also formed by cleaving, surface level exists at these end faces, as in the case of a red LD. It should be noted that there is a need to increase the output power of blue-violet LDs including those used as writing LDs in DVD drives. Increasing the output power of blue-violet LDs will result in a situation where the problem of COD can no longer be ignored. This means that blue-violet LDs also need to have a window structure to achieve high output power.
However, although Zn, Si, etc. can be used as impurities to form a window structure in LDs of GaAs-based material (having an oscillation wavelength of 780 nm) and in LDs of AlGaInP-based material (having an oscillation wavelength of 660 nm), they cannot be used to form an effective window structure in LDs of GaN-based material (having an oscillation wavelength of 405 nm) since the light absorption of the end faces will increase.